The present invention relates to the continuous vertical casting of metal or metal alloy tubes, and more particularly to the continuous casting of low thickness cast iron tubes using a die comprising a mold and a mandrel or cylindrical core, which is coaxial to the mold, and which defines an annular space for the casting of the tube. The invention is specifically concerned with the centering of the core or mandrel in the die.
The problem of centering the core in relation to the die is particularly acute in the casting of thin tubes having a low thickness/diameter ratio, whose thickness does not exceed a few millimeters, and particularly, as in the case of the present invention, tubes with a thickness/exterior diameter ratio of less than 8% and of a thickness of less than 5 mm. In effect, the slightest off-centering of the core in relation to the mold causes considerable irregularity in the thickness of the tube over the entire periphery of the die. For example, for a desired thickness of 3 mm, an off-centering of 0.5 mm causes a relatively important irregularity which can be from 1 to 3 mm, i.e., 33%. Such an off-centering can easily arise due to machining tolerances and/or slightly heterogeneous dilation of the core during heating.
As discussed in applicant's French Pat. No. 2,415,501, when the core is centered by adjusting it in an upper orifice of the casting head or box, the manufacturing tolerances of the casting head or box and the orifices intended to be traversed by the core are too wide for very low thicknesses, in particular those less than 5 mm, and the centering of the core is difficult to accomplish.
In accordance with another technique, known from French Pat. No. 1,485,055, an assembly with a sleeve or sheath and with a guiding bearing fitted by stamping into the sleeve or sheath provides precise centering of the mandrel in relation to the casting die of a tube. However, such a technique calls for precise adjustment which is difficult to achieve.
Precise adjustment is also called for if, as is known, a core with a centering range is used in the mold or shell.
In another prior art technique (French Pat. No. 2,077,405) using an ingot mold for the manufacture of tubular profiles, the problem of centering the mandrel in relation to the mold is resolved by inserting a cylindrical base of the mandrel in a corresponding cylindrical cavity of the mold with, in addition, a threading to axially immobilise the mandrel in the mold. In addition to the fact that this assembly comprises a threading which is difficult to machine, it does not provide a satisfactory solution to the problem of the continuous casting of thin cast iron tubes.